This invention relates to improvements in tandem master cylinders for hydraulic systems of the kind in which fluid under pressure is supplied to a primary outlet port from a primary pressure space in a bore between a positively actuated main piston and a floating or secondary piston, and to a secondary outlet port from a secondary pressure space between the floating or secondary piston and an end of a housing closing the bore, the position of primary piston with respect to the secondary piston being determined by the length in a released position of a return spring which act between the two pistons, the return spring being caged between the main piston and a sleeve in abutment with the secondary piston and which is coupled to the main piston through an axially extending headed stem to determine a defined position.
When the pistons are in their fully retracted positions, the two pressure spaces are in communication with a reservoir or reservoirs for fluid through recuperation valves which are controlled by the pistons, the valves being opened when the pistons are retracted and being closed when the main piston is advanced in the bore to apply the brakes.
To ensure simultaneous operation of brakes connected to each outlet port it is important that both valves close substantially simultaneously. To achieve such a simultaneous cut-off it is important that the travel of both pistons in a brake applying direction is substantially the same. Due to manufacturing tolerances the position to which the pistons have to be adjusted to achieve simultaneous cut-off has to be adjusted prior to assembly of the master cylinder.
The effective length of the return spring is therefore determined by the length of the stem and its effective length is adjusted during assembly to ensure that simultaneous operation of both valves takes place.
In GB2 075 631, which is hereby incorporated by reference herein, the effective length of the return spring is determined by coupling the stem to the main portion through a member which forms an abutment for the adjacent end of the spring and which is crimped to the stem when the effective length of the return spring has been determined. Specifically the member is crimped to engage grooves in the distal end of the stem.
DE 9421291, which is hereby incorporated by reference herein, discloses a similar construction. In this construction the stem is of hollow construction and the material of the main piston is deformed into grooves in the outer face of the hollow stem in the defined position.
In U.S. Pat. No. 6,012,288, which is incorporated by reference herein, the stem is pressed into a bore in the main piston.